PHARMACOLOGICAL CHARACTERIZATION OF THE 5-HYDROXYTRYPTAMINE RECEPTOR MEDIATING RELAXATION IN THE RAT ISOLATED ILEUM

1 The aim of the present study was to investigate a 5-HT4 receptor inv olvement in the mediation of a 5-HT-induced relaxation response in the rat isolated ileum in vitro. 2 Ileal segments were taken at regular i ntervals from the ileo-caecal junction to duodenum. 5-HT (1 mu M) indu ced a relaxation or contraction response in segments taken from the te rminal ileum: the relaxation decreased and finally disappeared as cont ractions dominated in the proximal tissues. The 5-HT-induced relaxatio ns were enhanced in the terminal segments and the contractions attenua ted in both terminal and proximal segments, in the presence of methyse rgide (1 mu M) and atropine (0.1 mu M). 3 In the presence of methyserg ide (1 mu M) and atropine (0.1 mu M), a cumulative addition of 5-HT (0 .01-1 mu M) induced a concentration-dependent relaxation in the termin al (1-20 cm from the ileo-ceacal junction) ileal segments which at hig her concentrations of 5-HT (3-30 mu M) reverted to contraction. 4 The rank order of potency of indole agonists in inducing a concentration-r elated relaxation response in tissues of the terminal ileum (pretreate d with pargyline (100 mu M) and in the presence of methysergide (1 or 100 mu M) and atropine (0.1 mu M)) was 5-hydroxytryptamine (6.97+/-0.0 6), 5-methoxytryptamine (6.50+/-0.07), alpha-methyl-5-hydroxytryptamin e (5.53+/-0.17), 5-carboxamidotryptamine (5.51+/-0.12) and 2-methyl-5- hydroxytryptamine (<5), the pEC(50), values (mean+/-s.e.mean) being sh own in parentheses. 5 Pretreatment of tissues with pargyline (100 mu M ) selectively enhanced the potency of 5-methoxytryptamine by a factor of 19 but failed to modify the potency of the other indole agonists. 6 The 5-HT4 receptor antagonists, tropisetron, SDZ 205-557 and GR 11380 8 antagonized the relaxation response to 5-HT (in the presence of meth ysergide (1 or 10 mu M) and atropine (0.1 mu M)) with pK(B) values (95 % CL) of 6.09 (5.94-6.24), 7.0 (6.9-7.09) and 8.95 (8.81-9.1) respecti vely. Apparent pK(B) values estimations for tropisetron (1 mu M) and G R 113808 (10 nM) using the agonists 5-methoxytryptamine and 5-carboxam idotryptamine were 6.37+/-0.31, 5.91+/-0.38 and 8.83+/-0.11, 8.82+/-0. 22 respectively. 7 Tropisetron (10 mu M), SDZ 205-557 (3 mu M) and GR 113808 (10-100 nM) caused an increase in basal tone of the rat termina l ileum when administered in the presence of methysergide and atropine . 8 The relaxation response to 5-HT in the rat terminal ileum was not antagonized by ritanserin (1 mu M), ondansetron (1 mu M) or N-omega-ni tro-L-arginine methyl ester (100 mu M) and with only a twofold dextral shift of the concentration-response curve by tetrodotoxin (1 mu M). 9 It is concluded that the relaxant response to 5-HT in the terminal re gion of the ileum is mediated directly at the smooth muscle; a ranked indole agonist potency and selective antagonism by 5-HT4 receptor anta gonists tropisetron, SDZ 205-557 and GR 113808 indicate a 5-HT4 recept or involvement in the relaxation response.